In a solid-state imaging device such as a CCD image sensor, CMOS image sensor or the like, electric charge proportional to the amount of received light is produced by photoelectric conversion by means of image pickup devices (photodiodes) in a plurality of pixels arranged on the imaging surface, and signal charge for each pixel is read, whereby an image on the light receiving surface is reproduced as an electrical signal.
In a photodiode, signal charge generated in proportion to the amount of light is read. Accordingly, the drawback that the dynamic range of the light intensity signal is narrow is theoretically exists. That is, although the signal charge increases linearly in proportion to the amount of light, there is a limit to the charge that can be accumulated in the photodiode of the photoelectric conversion section. By the limit of the accumulated charge, the maximum value of the dynamic range of the received image is determined. Further, when the signal charge is read in arbitrary resolution, the least significant bit (LSB) of the resolution determines the minimum value of the dynamic range. Furthermore, in the photodiode, generally a dark current exists. It is a general matter that due to variations in dark current between pixels, and temperature characteristics, information at the darkest time cannot be read by the accuracy up to 1 LSB which is the minimum sensitivity.
The problem of such saturation of charge or minimum sensitivity limit appears as a whitened or blackened image. The image information of a whitened part resulting from saturation has only information on saturated white, and details such as the gradation information on the brightness, in the case of the color, intensity ratios of adjacent pixels of three colors, that is, information on the color tone, and the like are lost. Further, in an image taken in color, when image information on each of one or two colors of adjacent three primary colors is saturated, and pixels of the remaining color or colors are not saturated, a color different from the actual color is represented. Further, the problem of the minimum sensitivity limit appears as a fact that information on the pixel of a certain noise level or lower is equally 0, i.e., as a blackened image. The image information of a blackened part has no information on the degree of darkness, and has only information indicating an equally black state of the level 0, and hence the details are lost.
The ordinary countermeasures against such a whitened image and blackened image lead to conflicting results. That is, in a certain image, when the aperture is reduced or the exposure time is shortened in order to prevent the whitening, a dark part becomes likely to be blackened further. Conversely, when the stop is opened or the exposure time is made longer in order to prevent the blackening, a light part becomes likely to be whitened further.
Such a phenomenon (drawback) is a point largely inferior to the film photography, and is a general phenomenon in imaging equipments utilizing the CCD image sensor or CMOS image sensor. In order to solve such a problem, and enlarge the dynamic range of the limited signal output, various methods have been proposed up to now.
Some measures presently advocated in order to enlarge the dynamic range of such signal output will be shown below.
(1) Dual sampling: Sampling is carried out two times by using different accumulation time lengths. Two images are combined, and an image with a wide dynamic range is obtained. This system essentially includes the problem of the simultaneity of signals, this becoming the weak point in imaging of a moving object. As a specific example to which dual sampling is applied, for example, a technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-288805 is known.
(2) Logarithmic conversion: Logarithmic conversion is carried out in the pixels by utilizing the fact that the drain current, and gate-source voltage exhibit logarithmic characteristics. In this system, there are problems of the responsiveness of a case where the optical signal is small, and exponential extension (the low level is exponentially enlarged, and hence fixed-pattern noise due to variations in dark level is enlarged) of a dark-time offset.
(3) Accumulation capacity modulation system: A system in which the Floating Diffusion (FD) capacity configured to accumulate signal charge in accordance with the intensity of the optical signal is modulated. In this system, the upper limit range of the dynamic range is physically determined by the upper limit of the capacitance value which is imparted to the FD capacity.
(4) In-pixel A/D converter and multiple sampling: A system in which an A/D conversion function is provided in the pixels, and resolution is changed according to the signal level by changing the accumulation time. In this system, there is a problem that a large number of elements must be arranged in the pixels.
(5) In-pixel analog processing system: The dual sampling system is carried out in the pixels by analog calculation. This system essentially includes, like the system of (1), the problem of the simultaneity of signals, this becoming the weak point in imaging of a moving object.
As described above, in the ordinary solid-state image device, the dynamic range is limited and, as a result of this, there is the problem that whitening or blackening occurs in the obtained image. Further, although in order to cope with such circumstances, several countermeasures are taken, each case has an advantage, and also has a drawback.